Metal-Alkene Complexes
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Metal-Alkene Complexes

Learn about the bonding and structural properties of metal-alkene complexes, including the D-C and MCP models. Understand how the alkene C=C bond length changes upon binding with metals.

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Questions and Answers

What is the reason for the increase in alkene C=C bond length on binding?

The C=C π bond is depleted by back donation from the metal and the M−alkene σ bond

What is the characteristic of the C−H bonds in strongly π basic Pt(0) complexes?

They are bent away from the metal to a much bigger extent

What is the effect of electron-withdrawing substituents on carbon on the M-(alkene) bond?

They strengthen the M-(alkene) bond

What is the difference between the D-C and MCP models?

<p>The D-C model is for weakly π-basic metals while the MCP model is for strongly π-basic metals</p> Signup and view all the answers

What is the length of the C=C bond in free C2H4?

<p>1.337 Å</p> Signup and view all the answers

What is the characteristic of the carbons in an MCP C2H4 complex?

<p>They are rehybridized from sp2 to sp3</p> Signup and view all the answers

What is the approximate angle that the C1 and C4 substituents twist out of the plane of the ligand?

<p>20–30°</p> Signup and view all the answers

What is the average length of the C1–C2 and C3–C4 distances?

<p>1.46 Å</p> Signup and view all the answers

Which molecular orbitals dominate the bonding to the metal?

<p>ψ2 and ψ3</p> Signup and view all the answers

What is the result of σ donation to the metal and back donation from the metal on the C1–C2 and C2–C3 distances?

<p>C1–C2 is lengthened, C2–C3 is shortened</p> Signup and view all the answers

At which carbon atom can protonation occur?

<p>C1</p> Signup and view all the answers

What is the result of M–L binding on the ligand HOMO and LUMO?

<p>The HOMO is depleted, the LUMO is populated</p> Signup and view all the answers

What happens to the metal in a fluxional η1-Cp group?

<p>It rapidly moves around the ring.</p> Signup and view all the answers

How many MOs are formed from the five carbon p orbitals in the MO scheme of Fig. 5.3?

<p>Five</p> Signup and view all the answers

Which metal orbital interacts with ψ1 in Fig. 5.3b?

<p>dz2</p> Signup and view all the answers

Why is the Cp group not a good π acceptor?

<p>It is a good π acceptor due to the anionic charge.</p> Signup and view all the answers

What must be considered in the MO diagram for a Cp2M metallocene?

<p>The sum and difference of each MO.</p> Signup and view all the answers

What is the effect of the anionic charge on Cp complexes?

<p>They become basic.</p> Signup and view all the answers

What is the purpose of using Et3O+ in complexes?

<p>To form a volatile by-product</p> Signup and view all the answers

What is the characteristic of the η8cyclooctatetraene (cot) hydrocarbon?

<p>Antiaromatic 8π electron</p> Signup and view all the answers

What type of complexes can early metals form with η8-C8H8?

<p>η8-C8H8 complexes</p> Signup and view all the answers

What is the term used to describe fragments that are very dissimilar in composition but have similar frontier orbitals?

<p>Isolobal</p> Signup and view all the answers

What is the characteristic of the frontier orbitals of isolobal fragments?

<p>Similar symmetries and electron occupations</p> Signup and view all the answers

What is the symmetry of the pair of ψ1 orbitals from each ring?

<p>a1g</p> Signup and view all the answers

What is the result of replacing one CH of benzene by a metal fragment isolobal with CH?

<p>A planar MC5 ring without alternating CC bond lengths</p> Signup and view all the answers

Which metal orbital does the opposite combination of ψ1 orbitals interact with?

<p>pz</p> Signup and view all the answers

What is the pattern of d-orbital splitting in Cp2M?

<p>Two-above-three pattern</p> Signup and view all the answers

Which metallocenes are the most stable?

<p>Group 8 metallocenes</p> Signup and view all the answers

What is the result of the bonding and nonbonding orbitals being exactly filled in Cp2Fe?

<p>The antibonding orbitals are empty</p> Signup and view all the answers

Why is the MCp2 unit intrinsically stable?

<p>The reason is not specified in the text</p> Signup and view all the answers

Study Notes

Experimental Structures

  • Experimental structures of metal-alkene complexes can fall between two extremes: Dewar-Chatt-Duncanson (D-C) model and Metal-Centered (MCP) model.
  • The D-C model fits best for intermediate oxidation state late metals, such as Zeise's salt, while the MCP model applies for Pt(0).
  • Both models are considered η2 structures.

Metal-Alkene Bonding

  • The alkene C=C bond length increases on binding due to two reasons:
    • M-alkene σ bond depletes the C=C π bond by donation to M, slightly weakening and lengthening the bond.
    • Back donation from the metal lowers the alkene C-C bond order by filling C=C π*.
  • For weakly π-basic Pt(II), the D-C model results in a slight reduction in bond length, while for strongly π-basic Pt(0), the MCP model applies, leading to a longer C=C bond length.

η2 Complexes

  • Electron-withdrawing substituents on carbon encourage back donation and strengthen the M-alkene bond.
  • The bonds to the four substituents of the alkene are bent away from the metal to a small extent in the D-C case, but to a much bigger extent in an MCP complex.
  • The C1 and C4 substituents twist 20-30° out of the plane of the ligand and bend back to overlap better with the metal.

Diene Complexes

  • The butadiene frontier orbitals, ψ2 (HOMO) and ψ3 (LUMO), dominate bonding to the metal.
  • The MO diagram shows that both depletion of electron density in ψ2 by σ donation to the metal and population of ψ3 by back donation from the metal lengthen C1-C2 and shorten C2-C3.
  • Protonation can occur at C1 where the HOMO, ψ2, has its highest coefficient.

Isolobal Replacement and Metallocycles

  • The chemical character of a fragment depends on the symmetry and electron occupation of the frontier orbitals.
  • Fragments that are very dissimilar in composition can have similar frontier orbitals, known as isolobal fragments.
  • Isolobality helps in understanding metallabenzenes, in which we replace one CH of benzene by a metal fragment isolobal with CH.

η8 Ligands

  • The antiaromatic 8π electron, nonplanar hydrocarbon, η8-cyclooctatetraene (cot), can form complexes as the reduced, aromatic 10πe cot2− dianion.
  • Early metals that need many electrons to achieve an 18e structure can also give η8-C8H8 complexes.

Cyclopentadienyl Complexes

  • The η1-Cp group can be fluxional, in which case the metal rapidly moves around the ring to make all the protons equivalent.
  • The MO scheme for M-C5H5 shows that the five carbon p orbitals lead to five MOs for the C5H5 group.
  • The most important overlaps are ψ1 with the metal dz2, and ψ2 and ψ3 with the dxz and dyz orbitals.

Metallocenes

  • The MO diagram for a Cp2M metallocene requires consideration of both Cp groups.
  • The bonding scheme retains both the L→M direct donation and the M→L back donation, as well as a d-orbital splitting pattern that broadly resembles the two-above-three pattern characteristic of an octahedral crystal field.
  • The MCp2 unit is intrinsically stable, and the same structure is adopted for numerous first-row transition metals, even when this results in a paramagnetic, non-18e complex.

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